http_proxy support

[holyjak]

slack-rtm currently doesn't support running behind a http(s) proxy. This has to be fixed in the upstream dependencies (gniazdo#16), this issue is here to make this deficiency visible and track upstream.

It might be fixed in Jetty 9.4.x but it seems to be moving extremely slowly :-(

(Note: clj-http supports proxies out of the box, using the default Java system properties http(s).proxyHost and http(s).proxyPort.)

[holyjak]

It seems that also http-kit now supports proxies and it has a websocket client so it could perhaps be used instead of gnizado.

CORRECTION I was wrong, http-kit doesn't have a websocket client yet though they are (have been?) working on it.